1. Field of the Invention
This invention relaes to an electronic amplifier circuit, in particular to an amplifier circuit based upon an operational amplifier and using switched capacitor circuits to replace resistor elements.
2. Description of the Prior Art
Many traditional amplifier circuit designs have not been suitable for fabrication as integrated circuits because certain components required in those designs, such as inductors, large capacitor values, or precise resistor values can not be easily obtained with standard metal oxide semiconductor (MOS) fabrication processes. Therefore, new designs have been developed to reduce the need for these difficult components. This invention furthers this trend, by replacing critical value resistors with switched capacitor circuits that can be easily integrated.
An amplifier circuit known in the art is shown in FIG. 1. This circuit uses an operational amplifier 12 with an inverting input terminal 14, a non-inverting input terminal 16, and a single output terminal 18. The input signal V1 passes through the input resistor R1 to the inverting input terminal 14. The non-inverting input terminal is connected to ground. The output signal at the output terminal 18 provides the amplifier circuit output signal at output node V2. The output signal is also returned through feedback resistor R2 to the inverting input terminal 14.
The gain and other response characteristics from the V1 node to the V2 node of this circuit are determined by the elements in the feedback path from the output terminal 18 back to the inverting input terminal 14. In particular, in the configuration shown, the voltage gain, V2/V1, of the circuit is set by the ratio of the feedback resistor R2 to the input resistor R1. Capacitors are sometimes used in the feedback path to control the frequency response of the circuit.
A problem with the direct implementation of this amplifier circuit as a MOS integrated circuit is that the precise value of resistors after the completion of the fabrication processes cannot be predicted. Therefore, it is difficult to insure that a precise ratio of R2 to R1 will be achieved. Therefore, the voltage gain of the circuit cannot be accurately predicted. This invention provides switched capacitor circuits to replace the resistor elements. The switched capacitor circuits are made of small capacitors and switching transistors. Small capacitors can be quite precisely manufactured due to the highly uniform thickness of oxide layers used as dielectric material, and the precise control of capacitor area provided by the fine masking methods used. The switching transistors can be precisely manufactured and will be controlled by a pair of clock signals. Therefore, the characteristics of the switched capacitor circuits can be predicted and controlled. This allows the manufacture of a final amplifier circuit of specific, stable, and predictable gain.
Switched capacitor circuits used switching transistors to control the application of signals to a set of capacitors. By controlling the frequency of switching action and the arrangement of switches and transistors, the rate of transfer of charge, frequency-selective characteristics, or digital sample and hold operations can be achieved.
Switched capacitor circuits have been used in the manufacture of band pass filters. They have also previously been used in the feedback loop of operational amplifiers, but not in the particular configuration and providing the valuable characteristics of this invention.
For example, FIG. 2 shows an amplifier circuit known in the art in which the resistors R1 and R2 of the operational amplifier circuit of FIG. 1 are replaced with standard switched capacitor circuits.
The input resistor R1 of FIG. 1 is replaced in FIG. 2 with switches 20 and 21, input capacitor C1, and switches 22 and 23. Feedback resistor R2 of FIG. 1 is replaced in FIG. 2 with switches 40 and 41, feedback capacitor C2, and switches 42 and 43.
The switches are implemented by any of several known configurations of MOS switching transistors. The simplest implementation as shown in FIG. 2 uses a single MOS transistor as a pass transistor. The switches are controlled by non-overlapping clock signals P1 and P2 as shown in FIG. 3, in order to alternately connect or ground the associated capacitor. When the capacitor is not connected in the signal path it is switched to ground to prevent accumulation of charge, which would prevent accurate following of the input signal. For example, during clock signal P1, switches 20 and 22 connect input capacitor C1 as the input path, and switches 40 and 42 connect feedback capacitor C2 as the feedback path. During clock signal P2, input capacitor C1 is grounded by switches 21 and 23, and feedback capacitor C2 is grounded by switches 41 and 43.
Since each capacitor will be passing signal only during periods when it is switched in the signal path, the total signal transfer will be reduced proportionately to the time it is switched out of the signal path. In this way, the switched capacitor circuit replaces the prior use of resistor elements.
Other examples of the use of switched capacitors to replace resistor elements are U.S. Pat. No. 4,404,525 to Amir et al., with switched capacitors in the input and feedback paths, and U.S. Pat. No. 4,441,080 to Saari, with switched capacitors in the feedback path.
Several problems arise with these prior solutions. First, the input and feedback signals are sampled only during periods when the capacitors are switched in. This results in a "stair-step" or aliasing type of waveform distortion, where a more linear version of the signals is desired.
Another problem is that these circuits drastically alter the characteristics of the feedback path when the feedback capacitor is switched out. The feedback path is simply open-circuitry for part of the time. This can cause drastic change in circuit operation. For example, if the feedback path is opened, a holding capacitor would be required to maintain the amplifier function. Parasitic or stray capacitance across the switching transistors can serve this function by default. However, this capacitance introduces frequency sensitive characteristics and adds a parasitic pole to the amplifier characteristics, essentially imposing a low-pass filter operation on the circuit.
This invention resolves both of these problems, avoiding stair step distortion, and maintaining stable feedback path characteristics.